In vitro correlation with in vivo effects, if achieved for one tumor system, is likely to hold as a valid biologic principle in other systems, including human breast cancer, even though sensitivity to individual drugs or combinations may vary greatly. Decreased ability of tumor cells to proliferate, as measured by a reduction in thymidine labeling index, and/or reduction in cell number under short-term assay conditions of tissue culture, may be sensitive in vitro correlates of significant tumor reduction in vivo. A test which measures these parameters in an animal model must also be one which is clinically applicable to the human situation: simple, rapid, reproducible, inexpensive, and requiring small numbers of tumor cells to carry it out.